The present invention relates to plasma processing systems. In particular, the present invention relates to plasma processing systems having capability of coil current ratio adjustment for controlling plasma uniformity.
Plasma processing systems are employed in various industries for fabricating devices on wafers. For example, the industries may include semiconductor, magnetic read/write and storage, optical system, and micro-electromechanical system (MEMS) industries. A plasma processing system may generate and sustain plasma in a plasma processing chamber to perform etching and/or deposition on a wafer such that device features may be formed on the wafer. In fabricating devices, it may be important to control plasma uniformity in order to satisfy certain production yield requirements and/or certain feature specifications. In general, plasma uniformity control may involve utilizing a power splitter having current adjustment capability, as discussed with reference to the example of FIG. 1.
FIG. 1 shows a schematic representation illustrating a cross-sectional view of an example prior art plasma processing system 100. Plasma processing system 100 may include a plasma processing chamber, which may include structural components such as a chamber wall 132, a pinnacle 130, a dielectric window 128, etc., for containing plasma, as illustrated by plasma 180. Inside the plasma processing chamber, plasma processing system 100 may include a chuck 136 (such as an electrostatic chuck) for supporting a wafer, as illustrated by wafer 134, during plasma processing.
Plasma processing system 100 may also include a radio frequency (RF) power source 170, an inner coil 126 disposed on dielectric window 128 and electrically coupled with RF power source 170, and an outer coil 124 electrically coupled with RF power source 170 and surrounding inner coil 126. Inner coil 126 and outer coil 124 may be disposed inside coil enclosure 138, which may be coupled to chamber wall 132. RF power source 170 may produce RF currents conducted by inner coil 126 and outer coil 124 for generating and sustaining plasma 180. For example, inner coil 126 may conduct a first RF current mainly for sustaining an inner portion of plasma 180 (near inner coil 126), and outer coil 124 may conduct a second RF current mainly for sustaining an outer portion of plasma 180 (near inner coil 124).
Plasma processing system 100 may also include a power splitter 112 for adjusting the RF currents conducted by inner coil 126 and outer coil 124, thereby controlling the uniformity of plasma 180. Power splitter 112 may be electrically coupled with RF power source 170 through a matching network 102. Power splitter 112 may include a variable capacitor 116 electrically coupled between RF power source 170 and inner coil 126 for adjusting the amperage of the first RF current, thereby adjusting the density of the inner portion of plasma 180. Power splitter 112 may also include another variable capacitor 120 electrically coupled between RF power source 170 and outer coil 126 for adjusting the amperage of the second RF current, thereby adjusting the density of the outer portion of plasma 180. By enabling separate adjustment of different portions of plasma 180, power splitter 112 may facilitate controlling the uniformity of plasma 180.
However, power splitter 112 may involve several disadvantages. For example, given that power splitter 112 requires two variable capacitors 116 and 120, costs for manufacturing, maintaining, and operating power splitter 112 may be substantially high. Currently, a variable capacitor (such as a variable vacuum capacitor) may cost more than 1,000 US Dollars; therefore, manufacturing power slitter 112 may cost more than 2,000 US Dollars. Moreover, each of variable capacitors 116 and 120 may include mechanical parts that may incur substantial maintenance and operating costs. In addition, each of variable capacitors 116 and 120 may require a step motor for actuating the mechanical parts to perform capacitance adjustment. The two step motors also may incur significant manufacturing, maintenance, and operating costs for power splitter 112. As a result, power splitter 112 may substantially increase the manufacturing, maintenance, and operating costs of plasma processing system 100.
The two variable capacitors and the two step motors may include a substantially large number of mechanical moving parts. The substantially large number of mechanical moving parts may introduce significant reliability problems in operating plasma processing system 100. Malfunction of any of the mechanical moving parts may negatively affect the plasma processing process and may lead to undesirable production yield.
In addition, given the limitations of existing commercially available capacitors, power splitter 112 may provide only a limited usable current ratio range. Operating plasma processing system 100 outside of the usable range may lead to unstable plasma, arcing, or failure of tuning by matching network 102; as a result, production yield requirements and/or device feature specifications may not be satisfied.